The impact of trace elements from plants on human nutrition: a case for biofortification

摘要

Women and children in the developing world consume largely plant-based diets, and are commonly deficient in iron and zinc. This is mainly due to the low bioavailability of iron and zinc from cereal and legume staples high in phytic acid (Zimmermann et al., 2005; Gibson et al., 2006). Zinc deficiency is also more common in areas of the world where the soil is low in zinc. Low soil levels of iodine and selenium also lead to foods low in these micronutrients and, in some countries, to widespread deficiencies. Iodine deficiency is widespread in mountain areas and river valleys where the soil has been leached of iodine. Selenium deficiency appears to be more focused on specific countries and has been reported in the Keshan region of China, New Zealand and some countries in Northern Europe. Iron, zinc, iodine and selenium are essential for human growth, mental development, and immune function, and, as requirements for these micronutrients are increased during infancy, childhood and pregnancy, these stages of the life cycle are more prone to deficiencies. Iron deficiency results in decreased work capacity (with the resulting economic consequences), compromised resistance to infection, poor cognitive development in young children, and poor pregnancy outcome, including maternal death if iron deficiency anemia is severe (Baynes et al., 1990). Zinc deficiency leads to growth stunting in children, intra-uterine growth retardation, poor sexual development in adolescents, increased susceptibility to infections, and mueosal atrophy. Cretinism, poor pregnancy outcome, and poor cognitive performance in school children are the major consequences of iodine deficiency, which may be exacerbated by iron and selenium deficiencies as both these micronutrients are required for thyroid hormone production (Zimmerrnann et al., 2006; Lyons et al., 2004). Selenium deficiency has been reported to lead to a cardiomyopathy (Keshan disease) and an osteoarthropathy (Kashin-Beck disease) and, as a component of the antioxidant enzyme glutathione peroxidase, selenium has been linked to protection against cancer initiation and to immune defense. Food fortification is often viewed as the most sustainable, cost-effective means to combat micronutrient deficiencies and iodine fortification of salt has eradicated iodine deficiencies in many parts of the world (Hurrell 1997, 2002). Selenium fortification of salt has also been successful to combat Keshan disease in China. Iron fortification of staple foods and condiments has had some success but is not applicable to rural populations which buy little or no processed foods, and constantly needs funds for the fortification compounds (Bouis, 2002). Zinc fortification at the national level has received little attention. Biofortification of staple plant foods could be a useful way to improve micronutrient nutrition for the rural poor. The development of micronutrient-dense staple food crops uses traditional breeding practices, soil fertilization or modem biotechnology to create self-fortifying plant foods (Zimmememn et al., 2002). Zinc, selenium and iodine levels in staple crops could be increased by soil fertilization. Iron and zinc contents, and their bioavailability, could be increased by plant breeding or genetic engineering. Although the range of iron and zinc levels in different varieties of cereals and legumes would allow a 2-3 fold increase in these micronutrients by plant breeding, these increases may still be below the amounts needed for nutritional adequacy. Genetic engineering, on the other hand, could provide native foods with levels of iron and zinc similar to those used in traditional food fortification. Iron and zinc concentration could perhaps be increased by manipulation of uptake and storage processes (e. g. expression of phytoferritin or metallothionein in cereal grains). Iron bioavailability could be improved by reducing phytate or